177 research outputs found
Dyson-Schwinger Equations: An Instrument for Hadron Physics
Dyson-Schwinger equations furnish a Poincare' covariant approach to hadron
physics. They reveal that dynamical chiral symmetry breaking is tied to the
long-range behaviour of the strong interaction and make predictions
corroborated by modern lattice-QCD simulations. A hallmark in the contemporary
use of DSEs is the existence of a nonperturbative, symmetry preserving
truncation that enables the proof of exact results. The systematic error
associated with the truncation's leading term has been quantified and this
underpins an efficacious one-parameter model that is being employed to study
meson excited states.Comment: 9 pages; LaTeX2e; Contribution to proceedings of "17th International
Conference on Few-Body Problems in Physics," Duke University/TUNL,
5-10/June/200
Confinement, DCSB, Bound States, and the Quark-Gluon Vertex
Aspects of the dressed-quark-gluon vertex and their role in the gap and
Bethe-Salpeter equations are briefly surveyed using an intuitive model. The
model allows one to elucidate why a linear extrapolation to the chiral limit of
extant lattice data on the dressed-quark mass-function overestimates this
function and hence the value of the vacuum quark condensate. The diagrammatic
content of the vertex described is explicitly enumerable. This property is
essential to the symmetry preserving study of bound state properties. It
facilitates a realistic analysis of vector and pseudoscalar meson masses, and
also allows the accuracy of standard truncations to be gauged. The splitting
between vector and pseudoscalar meson masses is observed to vanish as the
current-quark mass increases. That argues for the mass of the pseudoscalar
partner of the Upsilon(1S) to be above 9.4GeV. Moreover, in this limit the
rainbow-ladder truncation provides an increasingly accurate estimate of a bound
state's mass.Comment: 6 pages, Contribution to the Proceedings of "QCD Down Under", Special
Centre for the Subatomic Structure of Matter, University of Adelaide,
10-19/March/200
Looking into the matter of light-quark hadrons
In tackling QCD, a constructive feedback between theory and extant and
forthcoming experiments is necessary in order to place constraints on the
infrared behaviour of QCD's \beta-function, a key nonperturbative quantity in
hadron physics. The Dyson-Schwinger equations provide a tool with which to work
toward this goal. They connect confinement with dynamical chiral symmetry
breaking, both with the observable properties of hadrons, and hence provide a
means of elucidating the material content of real-world QCD. This contribution
illustrates these points via comments on: in-hadron condensates; dressed-quark
anomalous chromo- and electro-magnetic moments; the spectra of mesons and
baryons, and the critical role played by hadron-hadron interactions in
producing these spectra.Comment: 11 pages, 7 figures. Contribution to the Proceedings of "Applications
of light-cone coordinates to highly relativistic systems - LIGHTCONE 2011,"
23-27 May, 2011, Dallas. The Proceedings will be published in Few Body
System
Gluon Propagator on Coarse Lattices in Laplacian Gauges
The Laplacian gauge is a nonperturbative gauge fixing that reduces to Landau
gauge in the asymptotic limit. Like Landau gauge, it respects Lorentz
invariance, but it is free of Gribov copies; the gauge fixing is unambiguous.
In this paper we study the infrared behavior of the lattice gluon propagator in
Laplacian gauge by using a variety of lattices with spacings from
to 0.35 fm, to explore finite volume and discretization effects. Three
different implementations of the Laplacian gauge are defined and compared. The
Laplacian gauge propagator has already been claimed to be insensitive to finite
volume effects and this is tested on lattices with large volumes.Comment: RevTex 4.0, 14 pages, 9 colour figures; Correction to Reference
Scaling behavior of the overlap quark propagator in Landau gauge
The properties of the momentum space quark propagator in Landau gauge are
examined for the overlap quark action in quenched lattice QCD. Numerical
calculations are done on three lattices with different lattice spacings and
similar physical volumes to explore the approach of the quark propagator toward
the continuum limit. We have calculated the nonperturbative momentum-dependent
wave function renormalization function Z(p) and the nonperturbative mass
function M(p) for a variety of bare quark masses and perform an extrapolation
to the chiral limit. We find the behavior of Z(p) and M(p) are in reasonable
agreement between the two finer lattices in the chiral limit, however the data
suggest that an even finer lattice is desirable. The large momentum behavior is
examined to determine the quark condensate.Comment: 9 pages, 5 figures, Revtex 4. Streamlined presentation, additional
data. Final versio
Concerning the quark condensate
A continuum expression for the trace of the massive dressed-quark propagator
is used to explicate a connection between the infrared limit of the QCD Dirac
operator's spectrum and the quark condensate appearing in the operator product
expansion, and the connection is verified via comparison with a lattice-QCD
simulation. The pseudoscalar vacuum polarisation provides a good approximation
to the condensate over a larger range of current-quark masses.Comment: 7 pages, LaTeX2e, revtex
Strong Decays of Light Vector Mesons
The vector meson strong decays rho-->pi pi, phi-->KK, and K^star-->pi K are
studied within a covariant approach based on the ladder-rainbow truncation of
the QCD Dyson--Schwinger equation for the quark propagator and the
Bethe--Salpeter equation for the mesons. The model preserves the one-loop
behavior of QCD in the ultraviolet, has two infrared parameters, and implements
quark confinement and dynamical chiral symmetry breaking. The 3-point decay
amplitudes are described in impulse approximation. The Bethe--Salpeter study
motivates a method for estimating the masses for heavier mesons within this
model without continuing the propagators into the complex plane. We test the
accuracy via the rho, phi and K^{star} masses and then produce estimates of the
model results for the a_1 and b_1 masses as well as the mass of the proposed
exotic vector pi_1(1400).Comment: Submitted for publication; 10x2-column pages, REVTEX 4, 3 .eps files
making 3fig
Analysis of a quenched lattice-QCD dressed-quark propagator
Quenched lattice-QCD data on the dressed-quark Schwinger function can be
correlated with dressed-gluon data via a rainbow gap equation so long as that
equation's kernel possesses enhancement at infrared momenta above that
exhibited by the gluon alone. The required enhancement can be ascribed to a
dressing of the quark-gluon vertex. The solutions of the rainbow gap equation
exhibit dynamical chiral symmetry breaking and are consistent with confinement.
The gap equation and related, symmetry-preserving ladder Bethe-Salpeter
equation yield estimates for chiral and physical pion observables that suggest
these quantities are materially underestimated in the quenched theory: |<bar-q
q>| by a factor of two and f_pi by 30%.Comment: 9 pages, LaTeX2e, REVTEX4, 6 figure
Religious diversity, empathy, and God images : perspectives from the psychology of religion shaping a study among adolescents in the UK
Major religious traditions agree in advocating and promoting love of neighbour as well as love of God. Love of neighbour is reflected in altruistic behaviour and empathy stands as a key motivational factor underpinning altruism. This study employs the empathy scale from the Junior Eysenck Impulsiveness Questionnaire to assess the association between empathy and God images among a sample of 5993 religiously diverse adolescents (13â15 years old) attending state maintained schools in England, Northern Ireland, Scotland, Wales, and London. The key psychological theory being tested by these data concerns the linkage between God images and individual differences in empathy. The data demonstrate that religious identity (e.g. Christian, Muslim) and religious attendance are less important than the God images which young people hold. The image of God as a God of mercy is associated with higher empathy scores, while the image of God as a God of justice is associated with lower empathy scores
Survey of nucleon electromagnetic form factors
A dressed-quark core contribution to nucleon electromagnetic form factors is
calculated. It is defined by the solution of a Poincare' covariant Faddeev
equation in which dressed-quarks provide the elementary degree of freedom and
correlations between them are expressed via diquarks. The nucleon-photon vertex
involves a single parameter; i.e., a diquark charge radius. It is argued to be
commensurate with the pion's charge radius. A comprehensive analysis and
explanation of the form factors is built upon this foundation. A particular
feature of the study is a separation of form factor contributions into those
from different diagram types and correlation sectors, and subsequently a
flavour separation for each of these. Amongst the extensive body of results
that one could highlight are: r_1^{n,u}>r_1^{n,d}, owing to the presence of
axial-vector quark-quark correlations; and for both the neutron and proton the
ratio of Sachs electric and magnetic form factors possesses a zero.Comment: 43 pages, 17 figures, 12 tables, 5 appendice
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